Various approaches have been pursued to address the unsustainable annual generation and disposal of several hundred million metric tons of synthetic polymers, with the goal of a circular plastics economy. The use of renewable resources as feedstock materials generally does not address materials' end-of-use problems. The development of biodegradable polymers for biological recycling also provides a partial solution but fails to recover valuable building block chemicals. Degraded materials, especially those that only partially degrade, can also cause unintended environmental consequences. Mechanical reprocessing tends to degrade the quality of the polymers. In contrast, chemical recycling can allow for recovery of the precursor building block chemicals via depolymerization or creative reuse or repurposing through the generation of value-added materials.
With specifically designed monomers, reaction conditions can be used to select the direction of the monomer-polymer equilibrium or the closed-loop chemical cycle, with low temperatures and bulk or high monomer concentrations favoring polymerization and high temperatures or dilution triggering depolymerization. Several classes of recently designed recyclable polymers operate under this thermodynamic principle, such as poly[2-(2-hydroxyethoxybenzoate)], poly(β-methyl-6-valerolactone), and a polycarbonate (PC) derived from copolymerization of CO2 with a meso-epoxide. Poly[2-(2-hydroxyethoxybenzoate)] exhibited relatively low glass (˜27° C.), melting (˜69° C.), and decomposition (˜146° C.) temperatures; the thermostability of the PC was also limited (below 260° C.), and its depolymerization underwent decarboxylation.
However, the chemical recycling approach still faces challenges, including the selectivity involved in chemical recycling processes and circular monomer-polymer-monomer cycles, as well as trade-offs between polymers' depolymerizability and properties. A notable example for depolymerization selectivity is biodegradable poly(L-lactide) [P(L-LA)], which produces a mixture of many products upon thermolysis or a mixture of LA stereoisomers and cyclic oligomers upon chemolysis with a tin (Sn) catalyst, thus requiring substantial separation and purification before the recovered L-LA can be reused. Polymers with a low ceiling temperature (Tc) are readily depolymerizable under mild conditions, but they typically do not have robust enough physical and mechanical properties to be useful for most common applications. For example, poly(γ-butyrolactone) (PGBL), synthesized via catalyzed ring-opening polymerization (ROP) of the renewable, non-strained, thermodynamically highly stable five-membered γ-butyrolactone (GBL), can be selectively and quantitatively depolymerized back to GBL upon heating of the bulk material at 2600 or 300° C., depending on PGBL topology. However, the synthesis of PGBL requires energy-intensive, industrially undesirable low-temperature conditions (typically −40° C.), and PGBL exhibits limited thermostability and crystallinity, with a low melting transition temperature (Tm) of −60° C. Another example of a completely recyclable polymer was produced through the chemoselective ROP of bioderived α-methylene-γ-butyrolactone; however, not only was a low temperature (−60° C.) required for the polymer synthesis, but the resulting polymer was also a noncrystalline amorphous material.
Furthermore, the ring-opening polymerization (ROP) of cyclic esters or lactones is currently the most effective route for the chemical synthesis of technologically important, biodegradable and/or biocompatible aliphatic polyesters. However, this method is applicable only to common 4-, 6-, and 7-membered lactones with relatively high strain energy. The five-membered γ-butyrolactone (GBL) is a desirable monomer for the chemical synthesis of the corresponding biopolymer poly(γ-butyrolactone), PGBL, a structural equivalent of the microbial poly(4-hydroxybutyrate) (P4HB), which has been shown to exhibit several desirable properties for biomedical applications. Noteworthy also are the facts that GBL is a biomass-derived renewable monomer produced in a large industrial scale and PGBL can be completely recyclable back to its building block monomer in quantitative yield by simply heating the bulk material at 220° C. (for the linear PGBL) or 300° C. (for the cyclic PGBL) or in the presence of a catalyst at room temperature (Hong, M.; Chen, E. Y.-X. Nat. Chem. 2016, 8, 42-49). However, due to its low strain energy (i.e., high thermodynamic stability) of the five-membered lactone ring, GBL can only be ring-open polymerized under extreme conditions, such as ultrahigh pressure (e.g., 20,000 atm), into low molecular weight oligomers. Using powerful molecular catalysts, the ROP of GBL can take place under ambient pressure, but it requires very low temperature and high monomer concentration conditions (below −40° C.), producing PGBL with limited molecular weight (Mn up to 30 kg/mol). Typically, polymers are required to possess sufficiently high molecular weight to render them sufficient physical integrity and mechanical strength to be practically useful.
Accordingly, there is a need to discover new GBL-based monomers that not only can be polymerized under ambient conditions but can also lead to high molecular weight polyesters, while maintaining the chemical recyclability. Ideally, such monomers should be readily prepared from commercially available resources, can be polymerized under industrially convenient conditions (e.g., room temperature), and can produce the corresponding polyesters with relatively high molecular weight (Mn>100 kg/mol) and chemical recyclability (depolymerization back to monomer in high to quantitative selectivity).